1. Field
Embodiments of the disclosure relate generally to the field of supersonic aerodynamic inlets and more particularly to embodiments for generally axially symmetric chin or annular inlets with variable geometry provided by collapsing interleaved strips with alternate actuation.
2. Background
Inlet integration in supersonic cruise missile propulsion systems using gas-turbine engines is a significant challenge due to the broad operating speed range from subsonic to Mach 4. The additional requirements for light-weight, low-cost, and compactness due to launcher-imposed size constraints conflict with the need for high efficiency and low-drag which further exacerbates the difficulty in system design. Lower drag and higher efficiency benefits mission effectiveness through greater payload weight, longer range or higher speed.
An inlet needs to supply an airflow rate equal to or greater than that required by the engine. The excess flow must be either bypassed via a duct leading to the engine exhaust nozzle or through overboard exits, or spilled around the inlet cowl. Drag forces arise from the failure to employ the potentially captured flow to produce thrust from the engine. The state-of-the-art approach to supersonic missile inlet design usually involves a fixed-geometry design sized for the maximum capture area requirement set by the engine, and efficiency (total pressure recovery) optimized for the most critical thrust operating point. Certain variable geometry axisymmetric inlet concepts use either a translating or expanding centerbody. Alternative concepts include opening annular slots or other air inlet apertures to change the internal area contraction ratio for inlet “start” conditions.
Significant drag and efficiency penalties result at off-design flight conditions with fixed geometry inlets compared to what may be obtained with a variable geometry inlet. Excess air flow is usually bypassed around the engine with a drag penalty due to air flow energy loss associated with the inlet terminal normal shock and duct friction. Inlet internal contraction ratio, defined as the open area of the inlet divided by the area of the inlet throat, is limited by the ability to “start” at a certain Mach number. “Starting” can be defined as a stable condition in which the flow is supersonic at the cowl inlet lip and the terminal normal shock wave is located downstream of the inlet throat (i.e., the minimum cross-sectional area of the inlet duct).
An inlet has a limited internal contraction ratio at a given flight Mach number to allow self-start. Once started, the internal contraction ratio can be increased to a higher value. Increased internal contraction ratio results in a lower cowl angle (the angle of the interior and exterior surfaces of the inlet cowl relative to the horizontal) which decreases drag or increases efficiency for a given cowl angle.
Variable geometry inlet concepts use a mechanism that is relatively heavy, costly, complex and occupies significant volume. Such variable geometry inlets have high actuation forces which require bulky and expensive actuators. Such inlets also typically have trailing surfaces aft of the point of maximum diameter to provide a continuous surface with added associated weight and volume.
Two-dimensional inlet configurations have been provided for achieving the desired design efficiencies. However, some air vehicle concepts may require semi-cylindrical “chin” or “eyebrow” or full cylindrical axisymmetric inlets due to launcher interface dimensional constraints, or requirements for lower weight or drag.
It is therefore desirable to provide a variable geometry inlet with lightweight, low cost and reduced complexity for axisymmetric inlets.